[Bongle]

The three basic settings of a photo are ISO, aperture, and shutter speed.

ISO - Already discussed. Setting this higher increases the sensitivity of the sensor, but at the cost of introducing noise. The noise gets worse as your sensor size gets smaller, which is why DSLRs can take better high-ISO pictures than ultra-small compact cameras.
To get more light: Increase ISO
Tradeoff: More noise

Aperture - This is the size of the hole that the light travels through on its way to the image sensor. This is measured by a number called the f-stop. Unintuitively, a low f-stop means the aperture is MORE open, and a higher one indicates the aperture is LESS open. On my camera, it can go from 2.5 (wide open) to 8.0. Higher f-stops increase your depth of field, meaning the amount of stuff in focus increases. So with an f-stop of 8.0, you might have things between 2m away and 8m away in focus, but with an f-stop of 2.5, you may only have focus on things between 4-5m away. To take decent photos of robots, you probably want your aperture as open as your camera gets. This will 'highlight' the robot you want by making everything else slightly blurred, and will let in more light.
To get more light: Reduce f-stop (increase aperture size)
Tradeoff: Decreased depth of field (which actually kinda looks better anyway)

Shutter speed - Fairly obvious. Faster shutter speed = sharper photos = less light = darker photos. Slower shutter speed = more light = blurrier photos.
To get more light: Lower shutter speed
Tradeoff: Blurrier photos

Other techniques - Stand with the arena lights (if there are any) behind you. If they are in front, then you're going to get silhouetting of the robot, which looks awful. With the lights behind you, the photons go from the light to the robot to your camera, making for much better illumination.